Traditionally in radiographic imaging a beam of X-rays emitted by a source is directed through a subject to be examined, such as the body of a patient. The X-ray beam transmitted through the subject is detected by an X-ray detector and the resulting data provided by the X-ray detector is used to produce a representation in the form of an image of the internal structure of the subject. The absorption of the X-rays traversing the subject is linked to the density of the body tissues or the density of the material constituting an object under examination.
A difficulty often encountered in radiographic imaging of the human body, however, is the wide dynamic range of absorption of X-rays by the body. Curve 1 in the graph presented in FIG. 1 illustrates an example of the dynamic range of transmission of a typical beam of X-rays of 100 kV filtered by 2.5 of aluminium through a range of thicknesses of soft tissue. The range of tissue thicknesses under consideration may be extremely wide, especially in the case of an obese or overweight person. Typically the thickness of the abdomen is around 46 cm for a person of Body Mass Index (BMI) 46 kgm-2. The dynamic transmission range of the X-ray beam in a body can reach 3 to 5 decades depending on the BMI of the patient.
In digital radiography a radiographic image is constituted of pixels, each pixel corresponding to an element of detection or to a zone of detection. In order to generate an image, each detection pixel is provided with an electronic read-out which enables the electrical signal resulting from X-rays detected during a given integration time to be read. The electronic readout for each detection pixel usually includes a storage capacitor having a predefined charge storage capability. Typically each storage capacitor is dimensioned to be capable of storing the most intense charge signals, in particular those corresponding to a direct part of the X-ray beam (in which photons have been directly transmitted from the X-ray source to the X-ray detector without being absorbed by an object or body) from X-ray sources operating at a high energy (kVp) and high intensity (mAs). Such operating X-ray emission parameters are used, for example, in the radiographic imaging of corpulent bodies in which bone tissues located in deep anatomical zones of the body are being examined. The dynamic range of a quantum limited detector will be therefore defined by the ratio of the output signal corresponding to the direct X-ray beam, to the noise of the electronic readout. When the detector and readout have linear responses, the output signal is a linear function of the detector input signal. This means that anatomical areas of high absorption (for instance equivalent to 1 to 100 photons during the exposure time) having rich anatomical information, such as the bony structures for a musculoskeletal image, will be represented on the output image at the same finesse as areas of very high flux and low anatomical content, such as the skin. This is obviously not optimal for diagnostic needs and can lead to an increase in the dose administered to the patient.
During a radiography examination on a patient with low body corpulence or on an organ of low photon attenuation, the X-ray emission parameters are reduced to provide a less intense beam so that the dose received by the patient is minimised. In certain cases the intensity of the direct X-ray beam can be up to 200 times less than the intensity of the beam for the examination of bone tissues of a highly corpulent person. In these conditions the storage capacitor of the readout electronics only receives around 1/200 of the charge it is capable of accepting and the signal to noise ratio of the electronic read out chain is reduced by the same factor. If the detector or electronic readout is linear, this leads to a significant reduction in the contrast provided in the image and thus reduced image quality. In particular, anatomical areas of high absorption (for instance equivalent to 1 to 100 photons during the exposure time) will have an even lesser (200 times less) grey level range in the output image compared to the previous, high corpulence patient scenario. This is obviously detrimental to the image quality and leads to increasing the dose administered to the patient.